There is a big difference in low light when it comes to f2.8 and f1.8, so f1.4 would be even better, and for that alone I would consider prime lens.
But, I am not sure about everything else because 50mm f1.8 D for example is very soft below f5.6, and chromatic aberration is horrible, so it is much worse than my Sigma 17-50 f2.8 zoom, no matter it is a prime lens.
Of course, you can not compare cheap D lens with 50mm f1.4, but if you want to buy it for anything else than low light capabilities I think it is too expensive. But, that is subjective...
If this is my money, I will buy Sigma 18-35 f1.8, which is very sharp lens, so you get more at wide end and more light as well.